Aluminum is the preferred metal for VLSI interconnections because of its low resistivity, good adherence to Si and SiO.sub.2 layers, bondability, patternability, and ease of deposition (as a result of its low melting point). In addition, aluminum can be easily purified so that it does not contaminate the IC with undesirable impurities, and it is a readily available, low-cost material. In spite of its positive qualities, aluminum interconnections introduce many reliability problems, such as electromigration, contact failures, and step coverage.
Electromigration is one of the major interconnection failure mechanisms in VLSI integrated circuits. It is caused by the transport of the metal atoms when an electric current flows through the wire. This migration is a result of the interaction between the aluminum atoms and the electron current. As the atoms collide with the drifting electrons, the metal atoms are transported primarily by grain boundary diffusion. Because of its low melting point, aluminum has a large grain boundary self-diffusion constant, which increases its electromigration liability. When the metal atoms are displaced, the line may eventually break, and undesirable opens may be formed, or two neighboring lines can get shorted if material accumulated as a result of electromigration forms a bridge between the lines.
Electromigration-induced mass transport phenomena increase with current density and temperature. As the minimum feature size is scaled down, MTTF degrades rapidly because interconnect width and thickness are reduced and current density is increased.
There are a number of ways to minimize the electromigration rate. If the grain size is comparable to interconnection width and thickness, the electromigration rate is reduced because the self-diffusion path through grain boundaries is eliminated. An aluminum line with large grains can assume a bamboo-like structure. A more common way to reduce electromigration is the addition of alloying elements, which block the grain boundary diffusion path by precipitating at the grain boundary. Addition of 2 to 4 percent of copper to aluminum is shown to provide significant electromigration resistance. When silicon is alloyed with copper, however, it becomes difficult to dry etch, it corrodes more easily, its resistivity increases, and hillocks are not completely eliminated. Layered films, a recently developed technology in which layers of Al/Si are sandwiched between titanium or tungsten layers, promise to reduce electromigration, eliminate hillock formation, and produce low-resistivity lines that can be dry etched.
Regardless of the method of controlling electromigration, a method of determining when an interconnect line may be subject to electromigration is needed, which is provided by an aspect of the present invention.
Other objects and advantages will be apparent to those of ordinary skill in the art having reference to the following figures and specification.